1. Field of the Invention
The present invention relates to an optical illumination device for illuminating objects by light from a discharge lamp such as a mercury-arc lamp, and, more particularly, to a device suited for use in the optical illumination system of an exposure apparatus for manufacturing semiconductors.
2. Description of the Prior Art
Devices for illuminating objects using light from discharge lamps have been used in various fields in various applications. In reduction projection type exposure apparatus (steppers, aligners, etc.) used in photolithography for manufacturing LSI chips and other semiconductor or liquid crystal display elements, a device is used for illuminating reticles on which patterns for transfer are formed with a light beam of specified wavelengths (e.g., the i-line of a wavelength of 365 nm or the g-line of a wavelength of 436 nm) among the light radiated from an extra-high-pressure mercury-arc lamp (Hg lamp, Xe-Hg lamp, etc.).
In such projection exposure apparatus, however, efforts are continuing to transfer finer patterns at higher resolution to photosensitive substrates. In general, the resolution, R, and the depth of focus, DOF, of a projection exposure apparatus are expressed by the following equations: EQU R=k.sub.1 .multidot..lambda./NA (1) EQU DOF=k.sub.2 .multidot..lambda./NA.sup.2 ( 2)
where NA is the numerical aperture of the projection optical system in the projection exposure apparatus, .lambda. is the wavelength of light for exposure, and k.sub.1 and k.sub.2 are factors each decided by the process. It will be understood from the above equations that the finesse of the pattern is achieved by either one of the following methods:
(1) Increasing the numerical aperture (NA) of the projection optical system PA1 (2) Decreasing the wavelength of the light for exposure (exposure wavelength).lambda.. PA1 (1) First excited state: 105-180 nm PA1 (2) Second excited state: 180-240 nm PA1 (3) Third excited state: 260-340 nm PA1 (4) Fourth excited state: 340-390 nm
Of the two methods, projection optical systems with an NA of 0.5 to 0.6 or larger have recently been developed and resolution has been improved significantly. However, if the NA of a projection optical system is merely increased, as equation (2) shows, the DOF will decrease in inverse proportion to the square of the NA. In actual semiconductor manufacturing processes, in general, since circuit patterns must be projected onto a wafer having different levels produced in the preceding step and the flatness error of the wafer itself must also be absorbed, the DOF must be sufficiently large.
On the other hand, in the method of decreasing the exposure wavelength (.lambda.), as equation (2) shows, the DOF varies in proportion to the wavelength of exposure light. Therefore improving resolution R by decreasing the wavelength .lambda. of exposure light (as in the above method (2)) is more advantageous than improving resolution R by increasing the numerical aperture NR (as in the above method (1)) from the point of view of securing a large DOF. For these reasons, the bright-line called an i-line (a wavelength of 365 nm) emitted from a mercury-arc lamp is used more frequently in recent years than the conventionally used bright-line called a g-line (a wavelength of 436 nm) emitted from the same mercury-arc lamp.
FIG. 1 shows an example of a prior-art optical illumination device using a mercury-arc lamp as its light source used in a projection exposure apparatus. In FIG. 1, the luminescent point of a mercury-arc lamp 1 is located on the first focal point, F1, in a light reflecting and condensing member or elliptical mirror 2. On the top of the elliptical mirror 2 is formed an opening through which the electrode of the mercury-arc lamp 1 passes and, on the internal surface of the elliptical mirror 2, are vapor-deposited aluminum or various multilayer dielectric materials, thus causing the internal surface to function as a reflector. Light L emitted from the mercury-arc lamp 1 is reflected by the internal surface of the elliptical mirror 2 toward a mirror 3 for deflecting the optical path. Aluminum or various multilayer dielectric materials are also vapor-deposited on the reflecting surface of the mirror 3. Light reflected from the mirror 3 is collected on a second focal point, F2, of the elliptical mirror 2, thereby forming a light source image on the second focal point.
Divergent light from the light source image is converted to substantially parallel luminous flux by a collimator lens or input lens 4 and impinges on a narrow band-pass filter 5. The illuminating light of a wavelength selected by the band-pass filter 5 impinges on a fly-eye lens 6, which functions as an optical integrator, and many secondary light sources are formed on the focal plane behind the fly-eye lens 6 (on the reticle side). Divergent light from these many secondary light sources is reflected by a mirror 7 for deflecting the light path, converged by the condenser lens 8, and illuminates the pattern-formation surface of the reticle 9 on the surface to be illuminated so that light beams overlap. Aluminum or various multilayer dielectric materials are vapor-deposited on the reflecting surface of the mirror 7.
The entire optical system is compactly constructed by the use of mirrors 3 and 7 for deflecting the light path. The internal surface of the elliptical mirror 2 functioning as a convergent mirror, and the reflecting surfaces of the mirrors 3 and 7 are designed so that the reflectance of exposure light of a selected wavelength is maximized.
A super-high-pressure mercury-arc lamp is used as the mercury-arc lamp 1 in FIG. 1. The emission spectrum distribution of this super-high-pressure mercury-arc lamp is shown in FIG. 2. The wavelength dependence of the reflectance of the aluminum reflecting mirror having an aluminum vapor-deposited surface is shown in FIG. 3(a), and the wavelength dependence of the reflectance of a typical conventional dielectric multilayer reflecting mirror having a reflection surface on which multilayer dielectric materials are vapor-deposited is shown in FIG. 3(b). The wavelength dependence of transmissivity of the band-pass filter 5 in FIG. 1 when the exposure light is the i-line (a wavelength of 365 nm) is shown in FIG. 4. By such a structure, the i-line illumination light is selected, the pattern of a reticle 9 is illuminated by uniform illumination distribution, and the pattern of the reticle 9 is imaged through a projection optical system (not shown) onto the photosensitive substrate.
When a conventional optical illumination device as described above is operated under a condition open to the exterior, the surfaces of optical members (the elliptical mirror 2, the mirror 3, the collimator or input lens 4, and the band-pass filter 5 in FIG. 1) from the mercury-arc lamp 1 to the band-pass filter 5 are clouded, and reflectance and transmissivity gradually decrease, lowering illumination efficiency. This clouding phenomenon is known to be caused when clouding substances adhere to optical elements. The results of analysis using ion chromatography showed that the major substance causing clouding is ammonium sulfate ((NH.sub.4).sub.2 SO.sub.4).
The results of analysis using ESCA and SEM (scanning electron microscopy) showed that silicon oxides (SiO.sub.x) had also occasionally adhered to the surface of mirrors and lenses. If silicon oxides have adhered, scattering occurs if the surface is not smooth. Even if the surface is smooth, when silicon oxides have adhered to antireflection and high-reflection coating, the antireflection and reflection coatings are out of an optimum condition, and transmissivity or reflectance decreases, consequently lowering illumination. These clouding substances are considered to be ammonium ions (NH.sub.4.sup.+), sulfate ions (SO.sub.4.sup.2-), or organic silanol photochemically deposited optical elements where far ultraviolet (UV) rays impinge.
These ions or molecules of compounds have been considered to be those formed from the surface of materials such as black alumite (BAm) widely used in support or shield elements in illuminating optical systems, those originally existing in air, or those ionized by the irradiation of UV light. Since a diazo dye is used in BAm and sulfuric acid is used in the BAm process, BAm may be the sources of ammonium and sulfate groups, which are the constituents of ammonium sulfate. When UV light was irradiated onto a black alumite material in a nitrogen (N.sub.2) environment, oxygen (O.sub.2) environment, and normal atmospheric environment containing a large amount of water vapor, and the contamination of the optical element was compared, the largest amounts of ammonium groups and sulfate groups were formed in air containing water vapor, and the second largest amounts of these ions were formed in the (O.sub.2) environment. This proved that the possibility of accelerated contamination was heightened by the presence of water in air.
The irradiation of UV light to O.sub.2 is considered to cause O.sub.2 to be converted to ozone, which accelerates the formation of ions from the wall of the support element and the ionization of gases in air. However, the source of silicon oxides could not be found in the optical illumination system. When the dependence on the installation environment of the device was investigated, the presence of large amounts of ionic substances such as ammonium group, sulfate group, and nitrate group was found in a clean room, and, the presence of organic silanes such as hexamethyl disiloxane (HMDS) and trimethyl silanol was found. HMDS is a material widely used as a surface-treatment agent in applying photosensitive materials to wafers, and trimethyl silanol is formed when HMDS hydrolyzes. Investigation results showed a very close relationship between the amount of substances clouding optical elements and the amount of the above impurities. A detailed study on contamination showed that the source of clouding substances is present in the environment where the device is installed, but not in the device itself.
In filtering air passing through optical devices, a high-efficiency particulate air (HEPA) filter has conventionally been used. However, since the HEPA filter is for removing particles, it cannot remove ions or impurities which cause the photochemical reaction as described above.
As a method of avoiding the adherence of ammonium sulfate ((NH.sub.4).sub.2 SO.sub.4), U.S. Pat. No. 5,207,505 to the applicant of the present invention discloses a method for maintaining the optical device at a temperature of 120.degree. C. or more because the decomposition of ammonium sulfate begins at about 120.degree. C. (Kagaku Daijiten Vol. 9, p. 690, published by Kyoritsu Shuppan in 1964). However, although the light condensation mirrors, which are near the mercury-arc lamp, a large "heat source," can be maintained at a high temperature relatively easily, other optical elements require a considerably large separate heat source, and the dissipation of heat is very important in a semiconductor exposure apparatus which requires strict temperature control.
A prior-art device for preventing the clouding of optical elements (elliptic and direction-changing mirrors) by accommodating the light source and such optical elements in a container and introducing ion-particleless gas in this container is disclosed in Japanese Laid-Open Publication No. 4-139453. However, the device disclosed in this publication prevents only the formation of ammonium sulfate in the above optical elements. Furthermore, invention shown in this publication does not intend to prevent ammonium sulfate formation in optical elements disposed downstream side from the direction-changing mirror.
Meanwhile, the inventors of the present invention also examined the process in which ammonium sulfate ((NH.sub.4).sub.2 SO.sub.4) is formed from substances present in air in minute quantities. Results showed that a photochemical reaction in which light of a wavelength less than 5 nm is involved is assumed in an optical illumination device of prior art using the i-line (a wavelength of 365 nm) of a mercury-arc lamp shown in FIG. 1, because the adhesion of white ammonium sulfate powder is limited to within the area between the elliptical mirror 2 and the surface of the band-pass filter plate 5 on which light impinges.
Sulfur dioxide (SO.sub.2) and ammonia (NH.sub.3) are normally present in the air in very small amounts, and this is true also in a clean room in which a semiconductor exposure apparatus is operated. Therefore, the following reaction processes using oxygen (O.sub.2) and water (H.sub.2 O) in the air and the energy of UV light are considered.
(1) Sulfur dioxide (SO.sub.2) acquires the energy h.nu. (where h is Planck's constant) of UV light of a frequency .nu., to form activated sulfur dioxide (SO.sub.2.sup.): EQU SO.sub.2 +h.nu..fwdarw.SO.sub.2.sup. (i)
(2) The activated sulfur dioxide (SO.sub.2.sup.) is oxidized as the following formula to form sulfur trioxide (SO.sub.3): EQU 2SO.sub.2.sup. +O.sub.2 .fwdarw.2SO.sub.3 (ii)
(3) Sulfur trioxide (SO.sub.3) reacts with water as follows to form sulfuric acid (H.sub.2 SO.sub.4): EQU SO.sub.3 +H.sub.2 O.fwdarw.H.sub.2 SO.sub.4 (iii)
(4) Ammonia (NH.sub.3) reacts with water as follows to form ammonium hydroxide (NH.sub.4 OH): EQU NH.sub.3 +H.sub.2 O.fwdarw.NH.sub.4 OH (iv)
(5) The sulfuric acid (H.sub.2 SO.sub.4) formed in formula (iii) reacts with the ammonium hydroxide (NH.sub.4 OH) formed in formula (iv) (neutralization) as follows to form ammonium sulfate ((NH.sub.4).sub.2 SO.sub.4): EQU H.sub.2 SO.sub.4 +2NH.sub.4 OH.fwdarw.(NH.sub.4).sub.2 SO.sub.4 +2H.sub.2 O(v)
The above description refers to "Chiba University Environmental Science Research Report," Vol. 1, No. 1, pp. 165-177.
The inventors of the present invention noted that, if the reaction of formula (i) could be inhibited, formation of ammonium sulfate may be prevented. Therefore, if the reflectance of the light reflecting and condensating or converging element or member (2A) for the light having the light absorption band for sulfur dioxide is first decreased, the radiation of light activating sulfur dioxide decreases in subsequent optical elements, and, finally, the amount of ammonium sulfate, which causes white powder or white clouding to form, decreases. Thus, an optical illumination device which maintains a high illumination efficiency even in operation for a long period can be realized.
Next, the absorber of sulfur dioxide is described in detail. According to a different reference (H. Okabe, Photochemistry of Small Molecules, p. 248, Wiley-Inter-Science, 1978), sulfur dioxide has four absorption bands corresponding to the following first to fourth excited states:
Although the wavelength range of (1) and the wavelength range of (2) are continuous, and, similarly, the wavelength range of (3) and the wavelength range of (4) are continuous, these wavelength ranges or bands are separated because the electron states of excited sulfur dioxide formed by absorbing light of these wavelengths differ from each other. Therefore, if the light reflectance of the light reflecting and condensing or converging element (2A) for the light having at least one of the absorption bands from (1) to (4) is decreased, the radiation of light activating sulfur dioxide decreases in subsequent optical elements, and finally, ammonium sulfate formation, which causes white powder or white clouding to form, decreases.